As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
In addition, as interest in environmental problems is recently increasing, research into electric vehicles (EVs), hybrid EVs (HEVs), and the like that can replace vehicles using fossil fuels, such as gasoline vehicles, diesel vehicles, and the like, which are one of the main causes of air pollution, is actively underway. As a power source of EVs, HEVs, and the like, a nickel metal-hydride secondary battery is mainly used. However, research into lithium secondary batteries having high energy density, high discharge voltage and output stability is actively underway and some lithium secondary batteries are commercially available.
A lithium secondary battery has a structure in which an electrode assembly, in which a porous separator is interposed between a cathode and an anode, each of which includes an active material coated on a current collector, is impregnated with a lithium salt-containing non-aqueous electrolyte. As cathode active materials, lithium cobalt-based oxides, lithium manganese-based oxides, lithium nickel-based oxides, lithium composite oxides, and the like are mainly used. As anode active materials, carbon-based materials are mainly used.
However, in lithium secondary batteries using carbon-based materials as an anode active material, irreversible capacity occurs in some lithium ions intercalated into a layered structure of a carbon-based material during a 1st charging and discharging cycle and thus discharge capacity is reduced. In addition, carbon materials have a low oxidation/reduction potential of about 0.1 V with respect to potential of Li/Li+ and thus a non-aqueous electrolyte decomposes at an anode surface and such carbon materials react with lithium to form a layer coated on a surface of a carbon material (a passivating layer or a solid electrolyte interface (SEI) film). The thickness and boundary states of such an SEI film vary according to an electrolyte system used and thus affect charge and discharge characteristics. In addition, in secondary batteries used in fields that require high output characteristics, such as power tools and the like, resistance increases due to such an SEI film having a small thickness and thus a rate determining step (RDS) may occur. In addition, a lithium compound is produced at an anode surface and thus, as charging and discharging are repeated, reversible capacity of lithium gradually decreases and, accordingly, discharge capacity is reduced and cycle deterioration occurs.
Meanwhile, as an anode material having structural stability and good cycle characteristics, use of lithium titanium oxides (LTOs) is under consideration. In lithium secondary batteries including such LTOs as an anode active material, an anode has a relatively high oxidation/reduction potential of about 1.5 V with respect to potential of Li/Li+ and thus decomposition of an electrolyte hardly occurs and excellent cycle characteristics are obtained due to stability of a crystal structure thereof.
In addition, existing anode active materials are used by coating onto Cu foil, while an LTO may be used as an anode active material by coating onto Al foil.
However, it is difficult to distinguish a cathode including a cathode active material coated on Al foil from an LTO anode with the naked eye. In addition, Al lead can also be used and, accordingly, the LTO anode is mistaken for a cathode and thus positions of a cathode and an anode may be confused during module assembly or wiring for electrical connection.
Therefore, there is an urgent need to develop technology for fundamentally meeting such requirements.